In an electrical motor which has coil windings as part of its stator, such as in a switched reluctance electrical motor, it is common for the electrical motor to be powered by a three phase power supply and to apply AC waveforms to the coil windings to control the operation of the motor. Some electric motors have been created with more phases, but a significant factor in creating such multi-phase electrical motors is the cost of the phase driver electronics. In conventional motors a pulse width modulation (PWM) voltage controller (typically arranged with an H-bridge topology to drive motor coils bi-directionally) is coupled to a large power supply and the large voltages and currents which are required to drive the motor coils can add significantly to the cost of the driver circuitry. Accordingly, multiple phase motors requiring multiple coil driver circuits are expensive to produce according to contemporary techniques.
Also conventional motors usually only operate at their highest efficiency when operating at full design output power levels. It is known to provide variable output driver circuits to allow a motor to operate at reduced output power levels, but the motor efficiency is then markedly reduced due to a less effective coupling of the smaller magnetic fields. Consequently, to maintain reasonable efficiency such motors must be operated in narrow ranges near their design points, and mechanical gearboxes and transmission systems (even some with multiple motors) must be used to maintain efficiency across a wider range of output levels. Such configurations are both expensive and mechanically complex.